Nigel Farage at the European parliament in Strasbourg.
Photograph: Patrick Seeger/EPA

Nigel Farage has earned between £524,000 and £700,000 through TV and radio work in the last four years, according to transparency campaigners who are calling on MEPs to be more open about second jobs.

The former Ukip leader has the highest earnings outside the European parliament of any of Britain’s 73 MEPs, according to a report by Transparency International.

According to his own declarations, Farage has earned between €590,048 and €790,000 in the four years since the current parliament session began in July 2014. His exact earnings are unknown as MEPs are required to declare outside earnings only in broad bands.

Farage, who last year claimed to be “skint”, is only the seventh best paid MEP. In first place is the Italian socialist Renato Soru, who has earned €1.5m as director of Tiscali since June 2014. In second place is the centre-right Lithuanian Antanas Guoga, who has earned between €1.36m and €1.48m from online businesses and poker.

The third biggest earner is Guy Verhofstadt, the veteran Belgian MEP, who is co-ordinating the European parliament’s position on Brexit. Verhofstadt, a former prime minister of Belgium known for his passionate federalist speeches, earned between €920,000 and €1.45m for sitting on the board of the Belgian investment company Sofina and for appearances on the speaking circuit.

Almost a third of MEPs have second jobs or outside income, according to Transparency International, which is calling on MEPs to be obliged to provide more detail about their earnings and employers.

“There is no check, there is very little detail. It is all self-declared by MEPs,” said Daniel Freund, of the group. “All these side activities can be used to channel money and, given the absence of oversight, we simply don’t know if these are legitimate employment or whether this is a way of channelling illicit campaign donations to people.”

The group wants the parliament to set up an independent ethics body to vet MEP spending, replacing the current MEP-led system. In the last five years, 24 MEPs have been found to have breached the code of conduct, but none have been sanctioned for an ethics violation.

A spokesman for the Europe of Freedom and Direct Democracy group, which Farage leads, did not dispute the figures or reply to requests for more detail on the source of the earnings. “Mr Farage has declared his income in the financial declaration like every other MEP. Everybody is welcome to look at it,” the spokesman said.

A spokesman for Verhofstadt said he had “always declared his mandates openly and publicly on the website of the European parliament”.

The spokesman said Verhofstadt thought an independent ethics committee was a good idea. “He also stresses that he is not lobbying for companies and also believes that indeed there should be no lobbying by MEPs.”